1. Field of the Invention
This invention relates to hydraulic priority valves for preferential direction of fluid pressure, particularly for hydraulic devices on vehicles.
2. Description of the Prior Art
It is known to install priority valves when, on reasons of operating safety or reliability or in order to achieve a specific schedule of functions, one of two or more consuming devices must be suppled with pressure medium before an additional consuming device is connected to the source of that pressure medium. An example from the standpoint of operating safety or reliability might be a vehicle wherein a power brake and a level control are fed from a single pump. Clearly the brake, which is critically important to safety, must be given priority. An example from the standpoint of a schedule of functions might be the operating sequence (a) hydraulic gripping of a workpiece and (b) advance of a tool. Here clearly, the advance should only occur following the gripping.
A known priority valve (German laid-open application No. 23 56 235) employs a bypass valve which is hydraulically servo-controlled. The pressure medium flows from the pressure medium source through a check valve to a hydraulic accumulator and to the preferred consuming device. The pressure in the hydraulic accumulator serves as the control pressure for a preliminary control piston and an active surface of a main control piston, with the second active surface of said main control piston being relieved from or supplied with the control pressure depending on the state of charging of the hydraulic accumulator. The main control piston conveys the pressure medium stream to the non-preferred consuming device when the hydraulic reservoir is adequately charged.
The pressure-limiting valve may, at various times, be exposed to the pressure of the hydraulic accumulator. Such an arrangement is indeed functionally very reliable and is adequate for many applications. If, however, the preferred consuming device is required always to operate with the same pressure to the extent possible, as is desired for example in the case of a power brake with an accumulator, this cannot be brought about by a bypass valve. This stems from the fact that bypass valves ordinarily have switching-pressure differentials.
Accordingly the underlying problem of the invention is to devise a hydraulic priority valve of a suitable type wherein the consuming device which is preferentially supplied, and is in particular discontinuously operated, has the same pressure level available at all times. This pressure level should be, in particular, independent of any consumption on the part of other consuming devices. Said valve should be simple and inexpensive, with a minimum of parts, and in particular, should be suited to mass production. The installation space required should be small enough so that the entire assembly can be constructed as an attachment to a hydraulic accumulator.